Bibliographic details of the publications referred to in this specification are referenced at the end of the description. The reference to any prior art document in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that the document forms part of the common general knowledge in Australia.
High-density lipoproteins (HDLs) represent a broad group of mostly spheroidal plasma lipoproteins, which exhibit considerable diversity in their size, apolipoprotein (apo) and lipid composition. HDL particles fall into the density range of 1.063-1.21 g/ml (1) and as they are smaller than other lipoproteins, HDLs can penetrate between endothelial cells more readily allowing relatively high concentrations to accumulate in tissue fluids (2). The major apolipoprotein of almost all plasma HDLs is apo A-1, which in association with phospholipids and cholesterol, encloses a core of cholesteryl esters (1). Nascent (i.e. newly synthesised) HDLs secreted by the liver and intestine contain no cholesteryl esters and are discoidal in shape (1). The negative association of plasma HDL concentration with coronary artery disease has been well documented in epidemiological studies (3). Although experiments in animals have demonstrated an anti-atherogenic activity of HDLs (4), it is not yet known whether this protective effect is related to the role of the lipoprotein in reverse cholesterol transport or to a different mechanism. The mechanism/mechanisms via which HDLs provide these cardioprotective actions are not clearly understood, but may include a role for HDLs in reverse transport of cholesterol from peripheral tissues to the liver, inhibition of the oxidation of low-density lipoproteins, or modulation of vasodilatation and platelet activation mediated by changes in the production of prostacyclin (5). HDLs can also activate endothelial nitric oxide synthase subsequent to its interaction with scavenger receptor-B1 (SR-B1). Although HDLs are involved in the removal of cholesterol from extra-hepatic tissues, this subset of lipoproteins has recently been reported to possess functions unrelated to their role in plasma cholesterol transport. Almost 10 years ago, it was reported that in transgenic mice in which plasma levels of HDL were two-fold higher, the increase in plasma levels of TNF-α as well as mortality caused by bacterial lipopolysaccharide (LPS) were significantly reduced (6). Subsequently, it has been demonstrated that administration of native HDL or reconstituted HDL (=recHDL) significantly reduces organ injury and levels of mortality in animal models of endotoxic (LPS-mediated) and haemorrhagic shock (7). The beneficial actions observed in these models are—at least in part—mediated by the ability of HDLs to bind and inactivate LPS (6,8), directly inhibit expression of adhesion molecules on endothelial cells and via modulation of the expression of proinflammatory cytokines (6,9). In human volunteers, systemic administration of HDLs also downregulates the LPS ligand CD14 on monocytes and attenuates the release of TNF-α, IL-6 and IL-8 caused by small doses of intravenously administered LPS (10). HDL has also been shown to directly inhibit the TNF-α-induced expression of P-selectin on human endothelial cells (6). In addition, it has been reported that HDLs reduces the renal injury, dysfunction and inflammation caused by bilateral renal artery occlusion and reperfusion in the rat (11).
A growing body of data indicates that oxygen-derived free radicals such as superoxide (O2−), nitric oxide (NO) and hydroxyl radicals (OH−) have a role in mediating the intestinal damage in ischaemia/reperfusion [I/R] (12,13) as well as in inflammatory bowel disease (IBD) (14). The intestine is well endowed with enzymes capable of producing such free radicals (15). Moreover, when inflammation is present the many phagocytic cells that are attracted and activated can produce large amounts of free radicals. Several studies suggest that peripheral blood monocytes (16), and isolated intestinal macrophages (17), from patients with IBD produce increased amounts of free radicals. Also high numbers of peripheral polymorphonuclear leukocytes (PMNs), which are capable of producing large amounts of oxygen-derived free radicals (18), migrate into the intestinal wall of such patients (19). Grisham and Granger (20) hypothesised that—like in I/R injury—in ulcerative colitis transient ischaemic and subsequent reperfusion produce high levels of free radicals. This process initiates a cascade of events leading to the recruitment and activation of PMNs. In the last few years, various studies have gained substantial insight into the importance of specific adhesion molecules and mediators in processes, which finally result in the recruitment of PMNs at a specific site of inflammation. Activated PMNs, therefore, play a crucial role in the destruction of foreign antigens and the breakdown and remodelling of injured tissue. PMN-endothelial interactions involve a complex interplay among adhesion glycoproteins (i.e. integrins, members of the immunoglobulin superfamily and selectins). The firm adhesion of PMNs to the endothelium, however, is a complex phenomenon, which also involves other endothelium-based adhesion molecules. In fact, endothelial adhesion molecules are considered to play a pivotal role in the localisation and development of an inflammatory reaction (21). Intercellular adhesion molecule-1 (ICAM-1) is an adhesion molecule normally expressed at a low basal level, but its expression can be enhanced by various inflammatory mediators such as TNF-α and IL-1β (22).
Models of splanchnic artery occlusion shock (SAO) and 2,4,6-dinitrobenzene-sulfonic acid (DNBS)-induced colitis have been widely employed to investigate the pathophysiology of intestinal damage associated with I/R and with IBD. In work leading to the present invention, the inventors have investigated whether recHDL reduces the intestinal injury and inflammation caused by SAO shock and the chronic inflammatory response (colitis) caused by injection of DNBS in the rat. In order to highlight the possible mechanisms through which HDLs confer protection, the following endpoints of the inflammatory response have been determined: (1) PMN infiltration (determined as myeloperoxidase (MPO] activity, (2) pro-inflammatory cytokine production, (3) expression of adhesion molecules (i.e. ICAM-1), (4) lipid peroxidation (evaluated as malondialdehyde (MDA] levels) (5) peroxynitrite formation, (6) activation of the nuclear enzyme poly (ADP-ribose) (PAR) polymerase (PARP) and (7) morphological changes in the intestine.